1. Field of the Invention
The present invention relates to circuits for analog processing the signals output from image sensors such as CCD (Charge Coupled Device) area sensors and CMOS sensors, and imaging apparatuses provided with the same.
2. Description of the Related Art
FIG. 8 shows an example of a conventional analog circuit 10 for analog processing the output signals of an image sensor (see for example JP 2000-270267A and JP 2001-36358A). The analog circuit 10 is made of a noise reduction circuit 3 that receives signals output by a CCD 2, a gain variable amplifier 4 that receives signals output by the noise reduction circuit 3, an AD converter 5 that receives signals output by the gain variable amplifier 4 and converts them into digital signals that it outputs, a clamp circuit 6 for performing feedback control with respect to the digital signals output from the AD converter 5, a pulse control circuit 7 that supplies pulses for operating these component circuits, and a bias circuit 9 for supplying current to the noise reduction circuit 3, the gain variable amplifier 4, and the AD converter 5.
Four pulses are input to the pulse control circuit 7, namely a signal sampling pulse A for sampling the video period in the signals for each pixel at the horizontal drive frequency of the CCD, a reference sampling pulse B for sampling the reference voltage period in the signals for each pixel at the horizontal drive frequency, an AD converter pulse C for conversion to digital for each pixel at the horizontal drive frequency, and a clamp pulse D for fixing the DC voltage of the signals at the horizontal drive frequency. The pulse control circuit 7 creates and outputs various pulses for operating the other component circuits based on these pulses.
The noise reduction circuit 3 is operated in accordance with a pulse at the horizontal drive frequency that is output from the pulse control circuit 7 and reduces the noise in the output signals from the CCD 2. The gain variable amplifier 4 is operated in accordance with a pulse at the horizontal drive frequency that is output from the pulse control circuit 7 and adjusts the amplitude of the output signals of the noise reduction circuit 3 by altering the gain. The AD converter 5 is operated in accordance with a pulse at the horizontal drive frequency that is output from the pulse control circuit 7 and converts the output signals of the gain variable amplifier 4 into digital signals that it outputs. The clamp circuit 6 is operated in accordance with a pulse at the horizontal drive frequency that is output from the pulse control circuit 7 and performs feedback control such that the signal data of the reference period in the digital signals output from the AD converter 5 are identical to data that have been set freely.
The horizontal drive frequency, which is the repeat frequency of the signals output from the CCD 2, spans a wide range from several MHz to several 10 MHz due to the specifications for the number of pixels and the number of frames per second serving as video signals. The noise reduction circuit 3, the gain variable amplifier 4, and the AD converter 5 require a large amount of current if the horizontal drive frequency, which is the repeat frequency of the signals output from the CCD 2, is high, but are capable of operating adequately at less current if the horizontal drive frequency is low.
On the other hand, the output bias current, which is the current supplied to the noise reduction circuit 3, the gain variable amplifier 4, and the AD converter 5 from the bias circuit 9 of the conventional analog circuit 10 shown in FIG. 8, stays constant regardless of the frequency of the signals output from the CCD 2 that are to be processed. It is thus necessary that the current amount is set such that it is possible to process the output signals of a CCD 2 with the highest horizontal drive frequency specification that can be adopted. Consequently, a current amount that is greater than necessary is used if a CCD 2 with a lower horizontal drive frequency specification whose output signals can be processed with a smaller current amount is employed as is. Alternatively, it becomes necessary to redesign for a separate bias circuit that supplies a smaller current amount that is sufficient for this operation.
However, sending more current than is necessary shortens the charge life of the battery in battery-driven portable devices. Also, redesigning for a separate bias circuit results in an increase in the design time that is required to match the specifications of the CCD camera, for example.